Polarization multiplexed light transmitter and control method threof

ABSTRACT

The polarization multiplexed light transmitter takes out a part of a polarization multiplexed light to be transmitted as a monitor light; makes orthogonal polarization components contained in the monitor light to interfere with each other, to generate a polarization interfering light; converts the polarization interfering light into an electric signal; measures the power of an alternate current component contained in the electric signal after eliminating a direct current component thereof; and feedback controls delay amount varying sections so that an inter-polarized channel delay time judged based on a change in the measured power reaches a predetermined value. Thus, the delay time between the orthogonal polarization components in the polarization multiplexed light can be varied flexibly at a high speed with a simple configuration, and thus, it becomes possible to suppress transmission characteristics degradation of the polarization multiplexed light due to a change in system state.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2008-232413, filed on Sep. 10,2008, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discusses herein are directed to a polarizationmultiplexed light transmitter which applies a polarization multiplexingtechnology to transmit optical signals in optical communications and acontrol method thereof.

BACKGROUND

For realizing an ultrahigh speed optical transmission system of morethan 40 Gbit/s, investigations for a polarization multiplexingtechnology have been actively made. The polarization multiplexingtechnology is a format of multiplexing two orthogonal polarized signalsof same wavelengths, to transmit two independent signals. In thepolarization multiplexing technology, since two polarization states canbe utilized, it is possible to reduce a baud rate of transmission signaland to increase frequency utilization efficiency.

For an optical transmission system using this polarization multiplexingtechnology, there have been proposed various types of controltechnologies relating to polarization tracking mainly in a receptionsection (refer to Japanese Laid-Open Patent Application Publication No.2002-344426). It has been known that, in the polarization multiplexingoptical transmission system, degradation of transmissioncharacteristics, which is caused by a fiber nonlinear effect andpolarization mode dispersion (PMD), differs according to pulse timingbetween orthogonal polarization components in the polarizationmultiplexed light (refer to D. van den Borne, et. al., “1.6-b/s/HzSpectrally Efficient Transmission Over 1700 km of SSMF Using40×85.6-Gb/s POLMUX-RZ-DQPSK”, Journal of Lightwave Technology, Vol. 25,No. 1, pp. 222-232, Jan. 2007, 2).

To be specific, for pulse (bit) allocation between the orthogonalpolarization components in the polarization multiplexed light asillustrated in a left side of FIG. 30, in view of fiber nonlinear proofstrength, bit-interleaved polarization multiplexing which shifts pulsesof the orthogonal polarization components to each other by a half bit,achieves excellent transmission characteristics (refer to the right sideof FIG. 30). On the other hand, in view of PMD proof strength,bit-aligned polarization multiplexing which makes the pulse timingbetween the orthogonal polarization components in the polarizationmultiplexed light to be in-phase, achieves the excellent transmissioncharacteristics (refer to the center of FIG. 30). Accordingly, in orderto obtain desired transmission characteristics, it is necessary to setthe above pulse timing according to a state of the optical transmissionsystem.

However, in the conventional polarization multiplexing opticaltransmission system, the pulse timing between the orthogonalpolarization components is fixed when an apparatus for generating thepolarized multiplexed light is initially set. Accordingly, even though achange in system state, such as, time variation in the PMD, wavelengthpath modification, wavelength spacing modification or the like, iscaused, it is impossible to regulate the pulse timing between theorthogonal polarization components according to such a change.Therefore, there is a problem in that the transmission characteristicsare considerably degraded depending on the system state.

In order to solve the above problem and to enable the pulse timingbetween the orthogonal polarization components to be modifiable, theremay be considered, for example, a configuration in which a plurality oftransmitters corresponding to different pulse timings is previouslyprepared, and those transmitters are switched to be used according tothe system state. However, if the plurality of transmitters is disposed,there are drawbacks of large-scale of the apparatus, complexity thereofand high-cost performance thereof. Further, in a configuration whichmodifies the pulse timing between the orthogonal polarization componentsin one transmitter in manual according to the system state, themodifying work of pulse timing takes time, and therefore, in a casewhere the system state is changed quickly and frequently, it isdifficult to cope with such changes in system state.

SUMMARY

According to an aspect of the invention, a polarization multiplexedlight transmitter which transmits a polarization multiplexed lightcontaining a first optical signal and a second optical signal of whichpolarization states are mutually orthogonal, includes: a delayregulating unit configured to regulate relative delay times of the firstand second optical signals; and a delay control unit configured to makeorthogonal polarization components contained in a monitor light which isobtained by taking out a part of the polarization multiplexed light tointerfere with each other, to thereby generate a polarizationinterfering light, and configured to judge a delay time between thefirst and second optical signals based on a change in state of anelectric signal which is obtained by photo-electrically converting thepolarization interfering light, and configured to control regulationamounts in the delay regulating unit so that the judged delay timereaches a predetermined value.

Further, one aspect of a control method of the polarization multiplexedlight transmitter which transmits a polarization multiplexed lightcontaining a first optical signal and a second optical signal of whichpolarization states are mutually orthogonal, the control methodincluding: taking out a part of the polarization multiplexed light as amonitor light; making orthogonal polarization components contained inthe taken out monitor light, to interfere with each other, to therebygenerate a polarization interfering light; judging a delay time betweenthe first and second optical signals based on a change in a state of anelectric signal which is obtained by photo-electrically converting thegenerated polarization interfering light; and controlling relative delaytimes of the first and second optical signals so that the judged delaytime reaches a predetermined time.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a block diagram illustrating a configuration of a polarizationmultiplexed light transmitter in a first embodiment;

FIG. 2 is a block diagram illustrating a modified example relating to apolarization separating section and a polarization combining section;

FIG. 3 is a block diagram illustrating a first configuration example ofa polarization interfering section;

FIG. 4 is a block diagram illustrating a second configuration example ofthe polarization interfering section;

FIG. 5 is a block diagram illustrating a third configuration example ofthe polarization interfering section;

FIG. 6 is a block diagram illustrating a fourth configuration example ofthe polarization interfering section;

FIG. 7 is a block diagram illustrating a fifth configuration example ofthe polarization interfering section;

FIG. 8 is a diagram illustrating one example in which an opticalwaveform of a Pol-MUX_RZ_DQPSK signal light and an electric spectrumthereof are calculated;

FIG. 9 is a diagram illustrating one example in which an opticalwaveform of a polarization interfering light and an electric spectrumthereof are calculated, for the Pol-MUX_RZ_DQPSK signal light;

FIG. 10 is an enlarged diagram of the electric spectrum illustrated inFIG. 9 for when an inter-polarized channel delay time is 0 bit time, ¼bit time and ½ bit time;

FIG. 11 is a graph illustrating a relation of measured power to theinter-polarized channel delay time according to a monitor frequencyband;

FIG. 12 is a graph illustrating a relation of the measured power to theinter-polarized channel delay time for when a LPF of which 3 dBbandwidth is 300 MHz is applied;

FIG. 13 is a flowchart illustrating a specific example of feedbackcontrol by a delay control section;

FIG. 14 is a block diagram illustrating a configuration of a practicalexample 1-1 corresponding to the first embodiment;

FIG. 15 is a block diagram illustrating a configuration of a practicalexample 1-2 corresponding to the first embodiment;

FIG. 16 is a block diagram illustrating a configuration of a practicalexample 1-3 corresponding to the first embodiment;

FIG. 17 is a block diagram illustrating a configuration of a practicalexample 1-4 corresponding to the first embodiment;

FIG. 18 is a block diagram illustrating a configuration of thepolarization multiplexed light transmitter in a second embodiment;

FIG. 19A is a block diagram illustrating a configuration of a practicalexample corresponding to the second embodiment;

FIG. 19B is a block diagram illustrating a configuration of an appliedexample relating to the first and second embodiments;

FIG. 20 is a block diagram illustrating a configuration of thepolarization multiplexed light transmitter in a third embodiment;

FIG. 21 is a diagram illustrating changes in waveform of thepolarization interfering light according to the inter-polarized channeldelay time;

FIG. 22 is a block diagram illustrating a configuration of a practicalexample corresponding to the third embodiment;

FIG. 23 is a block diagram illustrating a configuration of thepolarization multiplexed light transmitter in a fourth embodiment;

FIG. 24 is a diagram illustrating changes in power of a frequencycomponent extracted by a synchronous detecting section according to theinter-polarized channel delay time;

FIG. 25 is a block diagram illustrating a configuration of a practicalexample corresponding to the fourth embodiment;

FIG. 26 is a block diagram illustrating a configuration of thepolarization multiplexed light transmitter in a fifth embodiment;

FIG. 27 is a graph illustrating a relation of the measured power to theinter-polarized channel delay time for when the LPF of which 3 dBbandwidth is 300 MHz is applied;

FIG. 28 is a block diagram illustrating another configuration relatingto the fifth embodiment;

FIG. 29 is a graph illustrating a relation of the measured power to theinter-polarized channel delay time in a NRZ-DQPSK modulation format; and

FIG. 30 is a diagram for explaining transmission quality degradation ofa polarization multiplexed light.

DESCRIPTION OF EMBODIMENTS

As described hereinbefore, the conventional polarization multiplexingoptical transmission system has a problem in that pulse timing betweenorthogonal polarization components is not adjustable according to achange in system state. As one of measures for coping with this problem,the applicant of this invention has proposed, in the prior JapanesePatent Application No. 2008-119011, a technology for: receiving, in areception unit, a polarization multiplexed light transmitted from atransmission unit to an optical transmission path; monitoringinformation relating to transmission characteristics of the receivedpolarization multiplexed light to transfer a monitoring result to thetransmission unit; and controlling, in the transmission unit, aninter-polarized channel delay time based on the transmissioncharacteristics information from the reception unit. This priorinvention is effective for solving the above problem, but aconfiguration of a control system is complicated. Further, a feedbackloop made up between the transmission unit and the reception unit islong, and therefore, a problem in that a control time is long stillremains.

Therefore, in order to solve the problem in the conventional technologyas well as the problem in the prior invention, this invention proposes apolarization multiplexed light transmitter capable of flexibly varyingat a high speed, with a simple configuration, a delay time between theorthogonal polarization components in the polarization multiplexedlight, and also, capable of suppressing transmission characteristicsdegradation of the polarization multiplexed light due to the change insystem state; and a control method thereof. There will be describedembodiments for implementing this invention, with reference to theaccompanying drawings. The same reference numerals denote the same orequivalent parts in all drawings.

FIG. 1 is a block diagram illustrating a configuration of a polarizationmultiplexed light transmitter in a first embodiment.

In FIG. 1, the polarization multiplexed light transmitter in the firstembodiment comprises: a light source section 11; a polarizationseparating section 12; a pair of optical modulating sections 13A, 13B: apair of modulation driving sections 14A, 14B; a polarization combiningsection 15; a delay regulating unit 20; and a delay control unit 30.Further, the delay regulating unit 20 includes a pair of delay amountvarying sections 21A and 21B, and the delay control unit 30 includes anoptical branching section 31, a polarization interfering section 32, aphotoelectric conversion section 33, a signal processing section 34, apower measuring section 35 and a delay control section 36.

The light source section 11 generates a continuous light or an opticalpulse train having a required wavelength, to output it to thepolarization separating section 12.

The polarization separating section 12 separates the output light fromthe light source section 11 to two lights of which polarization statesare mutually orthogonal, and outputs one of the two lights to theoptical modulating section 13A while outputting the other light to theoptical modulating section 13B.

To the optical modulating sections 13A and 13B, the lights polarizationseparated by the polarization separating section 12 are input, whileholding the polarization states thereof, and the optical modulatingsections 13A and 13B modulate the input lights in accordance withmodulation signals output from the modulation driving sections 14A and14B, and thus, generate first and second optical signals (sometimes tobe referred to as polarized channels, hereunder) corresponding to adesired optical modulation format, to thereby output the first andsecond optical signals to the polarization combining section 15. As themodulation format for the respective optical signals output from theoptical modulating sections 13A and 13B, it is possible to apply, forexample, an intensity modulation format such as NRZ (Non Return toZero), RZ (Return to Zero), CS-RZ (Carrier-Suppressed Return to Zero),Duobinary or the like; a phase modulation format such as QPSK(Quadrature Phase Shi Keying), DQPSK (Differential Quadrature PhaseShift Keying), DBPSK (Differential Binary Phase Shift Keying) or thelike; or a combination of the intensity modulation format and the phasemodulation format.

The modulation driving sections 14A and 14B generate the modulationsignals corresponding to the modulation format of the optical modulatingsections 13A and 13B, in accordance with transmission data fed from theoutside or the like, to supply them to the optical modulating sections13A and 13B via the delay amount varying sections 21A and 21B.

The polarization combining section 15 combines the orthogonal polarizedoptical signals output from the optical modulating sections 13A and 13Binto one light. The combined polarization multiplexed light passesthrough the optical branching section 31 in the delay control unit 30 tobe transmitted from an output port OUT of the present polarizationmultiplexed light transmitter to the outside.

Incidentally, herein, the configuration example is described in whichthe two polarization components are branched and combined using thepolarization branching section 12 and the polarization combining section15. However, this invention is not limited thereto, and it is alsopossible to apply a configuration in which an optical branching section16, polarization regulating sections 17A and 17B, and an opticalmultiplexing section 18 are combined with each other, as illustrated inan upper stage of FIG. 2. In this configuration, the output light formthe light source section 11 is branched into two lights by the opticalbranching section 16, and thereafter, the two branched lights arerespectively modulated by the optical modulating sections 13A and 13B,and the respective modulated lights are input to the correspondingpolarization regulating sections 17A and 17B. In the polarizationregulating sections 17A and 17B, polarization states of the twomodulated lights are regulated to be mutually orthogonal. As specificconfiguration examples of the polarization regulating sections 17A and17B, a λ/4 plate may be disposed to one of the polarization regulatingsections 17A and 17B while a −λ/4 plate being disposed to the otherthereof, to make the polarization states of the two modulated lights tobe mutually orthogonal. Or a λ/2 plate may be disposed to one of thepolarization sections 17A and 17B, to realize the orthogonalpolarization. Otherwise, variable polarization controlling devices maybe disposed to both the polarization regulating sections 17A and 17B andthe respective output lights are fed back, to thereby regulate thepolarization states of the two modulated lights to be mutuallyorthogonal. Then, the output lights from the polarization regulatingsections 17A and 17B are fed to the optical multiplexing section 18 sothat the polarization multiplexed light is generated. Further, it isalso possible to apply a configuration in which two light sources 11Aand 11B are used as the light source section 11 and output lights fromthe light sources 11A and 11B are supplied to the optical modulatingsections 13A and 13B, as illustrated in a lower stage of FIG. 2. In thiscase, it is possible to realize the light source section 11 by using twolow power light sources.

The optical branching section 31 branches a part of the polarizationmultiplexed light sent from the polarization combining section 15 to theoutput port OUT as a monitor light, to output the monitor light to thepolarization interfering section 32.

The polarization interfering section 32 makes the orthogonalpolarization components (polarized channels) contained in the monitorlight from the optical branching section 31 to interfere with eachother, to generate a polarization interfering light, and outputs thepolarization interfering light to the photoelectric conversion section33. It is possible to realize such a polarization interfering section 32by various types of configurations as illustrated in FIG. 3 to FIG. 7for example.

A first configuration example of the polarization interfering section 32illustrated in FIG. 3 includes a polarization angle regulator 321 and apolarizer 322. The polarization angle regulator 321 is fed with themonitor light from the optical branching section 31, and regulates apolarization angle of the monitor light so that polarization directionsof orthogonal polarized channels “x” and “y” in the monitor light aredeviated from a direction of main axis “a” of the polarizer 322.Incidentally, a z-direction illustrated by a broken line arrow in thefigure indicates an optical axis direction of the monitor light. Thepolarizer 322 is fed with the monitor light of which polarization angleis regulated by the polarization angle regulator 321, thereby generatinga light obtained by making the polarized channels “x” and “y” in themonitor light to interfere with each other, to output the polarizationinterfering light to the photoelectric conversion section 33. If adeviation angle of the polarization direction of the one polarizedchannel “y” relative to the direction of the main axis “a” of thepolarizer 322 is θ, this deviation angle θ is desirable to be 45°+360°×N(N is integer). By regulating the deviation angle θ to be 45°+360°×N, itbecomes possible to efficiently generate the polarization interferinglight.

A second configuration example of the polarization interfering section32 illustrated in FIG. 4 applies a polarization separating device 323 inplace of the polarizer 322 in the first configuration example of FIG. 3.Also in the second configuration example, in the polarization angleregulator 321, the polarization angle of the monitor light is regulated,so that the polarization directions of the orthogonal polarized channels“x” and “y” in the monitor light output from the optical branchingsection 31 are deviated from a direction of a main axis “a” of thepolarization separating device 323 (desirably, the deviation angle θ isto be 45°+360°×N). The polarization separating device 323 is fed withthe output light from the polarization angle regulator 321 to generate alight obtained by making the polarized channels “x” and “y” in themonitor light to interfere with each other, to thereby output thepolarization interfering light from first and second output portsthereof. Herein, the polarization interfering light output from thefirst output port of the polarization separating device 323 is sent tothe photoelectric conversion section 33, and an optical terminator 324is connected to the second output port of the polarization separatingdevice 323.

Incidentally, in the first and second configuration examples of theabove polarization interfering section 32, the explanation has been madeon the case where the polarization angle of the monitor light isregulated using the polarization separating device 323. However, inplace of the polarization separating device 323, a branching port of theoptical branching section 31 may be connected via apolarization-preservation fiber to an input port of the polarizer 322 orthe polarization separating device 323, to thereby add required angledeviation when the polarization-preservation fiber is splice connectedto the respective ports.

In a third configuration example of the polarization interfering section32 illustrated in FIG. 5, in place of the optical terminator 324 in thesecond configuration example of FIG. 4, a polarization angle regulator325 is connected to the second output port of the polarizationseparating device 323, and further, a polarization combining device 326is disposed for combining an output light from the polarization angleregulator 325 with the output light from the first output port of thepolarization separating device 323. In the third configuration example,the polarization angle of the monitor light is regulated by thepolarization angle regulator 321, so that the polarization direction ofone of the orthogonal polarized channels “x” and “y” in the monitorlight output from the optical branching section 31 (herein, thepolarization direction of the polarized channel “y”) is coincident withthe main axis “a” of the polarization separating device 323. The outputlight from the polarization angle regulator 325 is fed to thepolarization separating device 323, so that the polarized channel “y” isoutput from the first output port of the polarization separating device323, whereas the polarized channel “x” is output from the second outputport of the polarization separating device 323. The polarizationdirection of the polarized channel “x” output from the second outputport of the polarization separating device 323 is rotated by 90° in thepolarization angle regulator 325, to be in the same polarizationdirection as that of the polarized channel “y”. Then, the polarizedchannel “y” output from the first output port of the polarizationseparating device 323 and the polarized channel “x” of whichpolarization direction is rotated by 90° by the polarization angleregulator 325, are input to the polarization combining device 326, sothat the light obtained by making the polarized channels “x” and “y” tointerfere with each other is generated, and the polarization interferinglight is sent to the photoelectric conversion section 33.

In a fourth configuration example of the polarization interferingsection 32 illustrated in FIG. 6, in place of the polarization angleregulator 325 used in the third configuration example of FIG. 5, afaraday rotating mirror (FRM) 327 is disposed. The FRM 327 has opticalcharacteristics of rotating a polarization angle by 90° when reflectingan incident light. Herein, the polarized channel “x” output from thesecond output port of the polarization separating device 323 isreflected from the FRM 327, so that the polarization direction of thepolarized channel “x” is made to be same as that of the polarizedchannel “y”. As a result, similarly to the third configuration example,the light obtained by making the polarized channels “x” and “y” tointerfere with each other is generated in the polarization combiningdevice 326, and the polarization interfering light is sent to thephotoelectric conversion section 33.

A fifth configuration example of the polarization interfering section 32illustrated in FIG. 7 includes an optical amplifier 328 and a nonlinearelement 329. The optical amplifier 328 receives the monitor light fromthe optical branching section 31, and amplifies the power of the monitorlight up to a level at which a nonlinear effect can be generated in thelatter staged nonlinear element 329, to output the amplified monitorlight. The nonlinear element 329 receives the monitor light amplified bythe optical amplifier 328 and a control light fed from the outside orthe like, and similarly to an operating principle of a known opticalKerr switch, rotates the polarization directions of the polarizedchannels in the monitor light, by cross phase modulation (XPM) which isone of nonlinear effects. For example, the polarization direction of thepolarized channel “x” in the monitor light is rotated by a predeterminedangle φ, by XPM generated in the nonlinear element 329, so that acomponent, which is parallel to the polarized channel “y”, in therotated polarized channel “x”, and the polarized channel “y” interferewith each other, and consequently, the polarization interfering light issent to the photoelectric conversion section 33 from the nonlinearelement 329.

Incidentally, the first to fifth configuration examples for thepolarization interfering section 32 have been recited. However, thepolarization interfering section to be used in the invention is notlimited to the above configuration examples, and it is possible to applyarbitrary configurations capable of making the orthogonal polarizedchannels “x” and “y” in the monitor light to interfere with each other.

The photoelectric conversion section 33 (FIG. 1) receives thepolarization interfering light output from the polarization interferingsection 32, and converts the input light into an electric signal tooutput it.

The signal processing section 34 eliminates at least a direct current(DC) component from the electric signal output from the photoelectricconversion section 33, to extract an alternate current (AC) component.

For a frequency band of the alternate current component extracted by thesignal processing section 34, by ensuring a broader band thereof, itbecomes possible to increase a measured power in the latter staged powermeasuring section 35. However, as described later in detail, since aninter-polarized channel delay time can be judged only by monitoring achange in power of a part of the alternate current component of thepolarization interfering light, it is also possible to restrict thefrequency band of the alternate current component extracted by thesignal processing section 34. In the case where a high-frequency circuitis applied to a monitoring system, such an application increases a sizeof the polarization multiplexed light transmitter and a cost thereof.Therefore, in view of the miniaturization and low-cost performance ofthe polarization multiplexed light transmitter, it is desirable todecrease the frequency band of the alternate current component extractedby the signal processing section 34, and to be specific, it ispreferable to restrict the frequency band to a lower side of a baud rateof modulation signal. It is possible to make up such a signal processingsection 34 by a combination of a capacitor for eliminating the directcurrent component and a low-pass filter (LPF) or by using a band-passfiler (BPF).

The power measuring section 35 measures the power (the total power inthe frequency band) of the alternate current component extracted by thesignal processing section 34, to transfer a measurement result to thedelay control section 36.

The delay control section 36 judges the inter-polarized channel delaytime based on a change in the power measured by the power measuringsection 35, to generate control signals for feedback controlling delayamounts of the delay amount varying sections 21A and 21B.

The delay amount varying section 21A feeds a variable delay amount tothe modulation signal which is to be supplied to the optical modulatingsection 13A from the modulation driving section 14A. Further, the delayamount varying section 21B feeds a variable delay amount to themodulation signal which is to be supplied to the optical modulatingsection 13B from the modulation driving section 14B. The respectivedelay amounts of the delay amount varying sections 21A and 21B arecontrolled in accordance with the control signals from the delay controlsection 36. Incidentally, herein, the delay amount varying sections 21Aand 21B are individually disposed corresponding to the respectiveoptical modulating sections 13A and 13B. However, it is only necessarythat a relative delay time between the modulation signals fed to theoptical modulating sections 13A and 13B is variable, and therefore, itis possible to omit one of the two delay amount varying sections 21A and21B.

Next, there will be described an operation of the first embodiment.

In the polarization multiplexed light transmitter of the aboveconfiguration, the output light from the light source section 11 isseparated, by the polarization separating section 12, into two lights ofwhich polarization states are mutually orthogonal. Thereafter, theseparated two lights are modulated by the optical modulating sections13A and 13B, and the respective modulated lights are combined by thepolarization combining section 15 so that the polarization multiplexedlight is generated. This polarization multiplexed light is transmittedto an optical transmission path (not illustrated in the figure) or thelike connected to the output port OUT, and also, a part thereof isbranched by the optical branching section 31 as the monitor light, to befed to the polarization interfering section 32.

In the polarization interfering section 32, the light obtained by makingthe orthogonal polarized channels “x” and “y” in the monitor light tointerfere with each other is generated. This polarization interferinglight is converted into the electric signal by the photoelectricconversion section 33. Thereafter, the alternate current component inthe electric signal is extracted by the signal processing section 34,and the power of the alternate current component is measured by thepower measuring section 35, and further, the measurement result istransferred to the delay control section 36.

In the delay control section 36, the inter-polarized channel delay timeis judged based on the measurement result in the power measuring section35, namely, the change in the power of the alternate current componentin the electric signal obtained by photo-electrically converting thepolarization interfering light.

Here, there will be described in detail a judging method of theinter-polarized channel delay time in the delay control section 36. Inthe following description, it is assumed that the RZ-DQPSK modulation isperformed in each of the optical modulating sections 13A and 13B, and asignal light (to be expressed as Pol-MUX_RZ-DQPSK signal light,hereunder) obtained by polarization multiplexing RZ-DQPSK signal lightsgenerated in the optical modulating sections 13A and 13B by thepolarization combining section 15, is transmitted. Note, a bit rate ofthe Pol-MUX_RZ-DQPSK signal light is set at 43 Gbps.

FIG. 8 is one example in which an optical waveform of thePol-MUX_RZ-DQPSK signal light transmitted from the output port OUT ofthe polarization multiplexed light transmitter and an electric spectrumthereof are calculated according to the inter-polarized channel delaytime. An upper stage of FIG. 8 indicates the optical waveform, a mediumstage thereof indicates the electric spectrum of the alternate currentcomponent for when the frequency band is restricted to the level up to50 GHz, and a lower stage thereof indicates the electric spectrum inwhich the frequency band equal to or lower than 2 GHz is enlarged.

It is understood from FIG. 8 that, if the delay time between theRZ-DQPSK signal lights (polarized channels) generated by the opticalmodulating sections 13A and 13B is changed, the optical waveform of thePol-MUX_RZ-DQPSK signal light transmitted from the polarizationmultiplexed light transmitter is varied, and therefore, when the opticalwaveform for when the delay time is 0 bit time (time slot) or 1 bit timeis compared with the optical waveform for when the delay time is ½ bittime, there is a remarkable difference therebetween. On the other hand,a significant variation does not occur in the electric spectrum of thePol-MUX_RZ-DQPSK signal light, even if the inter-polarized channel delaytime is changed. Namely, it is difficult to judge the inter-polarizedchannel delay time only by directly photo-electrically converting thePol-MUX_RZ-DQPSK signal light transmitted from the polarizationmultiplexed light transmitter to monitor the electric spectrum thereof.Therefore, in the present embodiment, the optical branching section 31takes out the part of the polarization multiplexed signal lighttransmitted from the polarization multiplexed light transmitter as themonitor light, and the polarization interfering section 32 makes theorthogonal polarized channels in the monitor light to interfere witheach other, to thereby generate the polarization interfering light, sothat the inter-polarized channel delay time can be judged based on achange in an electric spectrum of the polarization interfering light.

FIG. 9 is one example in which, for the Pol-MUX_RZ-DQPSK signal light inFIG. 8, an optical wave of the polarization interfering light obtainedby making the orthogonal polarized channels to interfere with each otherand the electric spectrum thereof are calculated. Further, FIG. 10illustrates the enlarged electric spectrum (the alternate currentcomponent up to 50 GHz) in FIG. 9 for when the inter-polarized channeldelay time is 0 bit time, ¼ bit time and ½ bit time.

As illustrated in FIG. 9 and FIG. 10, for the electric signal obtainedby photo-electrically converting the polarization interfering light, thetotal power of the alternate current component is decreased if theinter-polarized channel delay time is increased from 0 bit time to ½ bittime. Further, if the inter-polarized channel delay time is increasedfrom ½ bit time to 1 bit time, the total power of the alternate currentcomponent is increased. Namely, the power of the alternate currentcomponent in the electric signal obtained by photo-electricallyconverting the polarization interfering light becomes maximal when theinter-polarized channel delay time is n (n is integer) bit time, whilebecoming minimal when the inter-polarized channel delay time is (2n+1)/2bit time.

Further, the power of the alternate current component in the electricsignal obtained by photo-electrically converting the polarizationinterfering light is changed as illustrated in FIG. 11 for example, ifthe frequency band of the alternate current component to be monitored ischanged. According to calculation results in FIG. 11, as the monitoredfrequency band (represented by a ratio to the baud rate of signal lightin FIG. 11) becomes broader, the monitored power of the alternatecurrent component is increased. However, a relation of the monitoredpower to the inter-polarized channel delay time becomes maximal when theinter-polarized channel delay time is 0 bit time or 1 bit time, whilebecoming minimal when the inter-polarized channel delay time is ½ bittime, regardless of the width of the monitored frequency band.Accordingly, as described before, it is also possible to judge theinter-polarized channel delay time based on the measured power in thepower measuring section 35, by restricting the monitored frequency bandto a low frequency alternate current component by the signal processingsection 34 to achieve the miniaturization and low-cost performance ofthe polarization multiplexed light transmitter.

FIG. 12 is a diagram in which the relation between the power measured bythe power measuring section 35 and the inter-polarized channel delaytime is obtained by calculation, when the combination of the capacitorfor eliminating the direct current component and the low-pass filter(LPF) of which 3 dB bandwidth is 300 MHz is applied. As illustrated inFIG. 12, it is also possible to judge the inter-polarized channel delaytime based on the change in the measured power, by restricting themonitored frequency band to the low band of 300 MHz.

As a result of focusing on the characteristics of the polarizationinterfering light as described above, the delay control section 36 mayfeedback control the delay amount varying sections 21A and 21B so thatthe measured power becomes minimal, when the inter-polarized channeldelay time is set to be bit-interleaved, whereas the delay controlsection 36 may feedback control the delay amount varying sections 21Aand 21B so that the measured power becomes maximal, when theinter-polarized channel delay time is set to be bit-aligned. Further, itis also possible to set the inter-polarized channel delay time to be inbetween the bit-interleave and the bit-align. In this case, the delaycontrol section 36 may feedback control the delay amount varyingsections 21A and 21B, after performing an offset based on a relationbetween maximal and minimal. Namely, by applying this invention, itbecomes possible set the inter-polarized channel delay time (pulsetiming) at a desired value.

Incidentally, it is possible to realize the above feedback controlrelatively easy by superimposing the low frequency signal (ditheringsignal) on at least one of the modulation signals fed to the opticalmodulating sections 13A and 13B to observe a change amount in adithering component contained in the monitoring result.

As one specific example of the feedback control by the delay controlsection 36, FIG. 13 illustrates a flowchart for when the inter-polarizedchannel delay time is set to be either bit-interleaved or bit-aligned.Firstly, the delay control section 36 is fed with an instruction as towhich of bit-interleave and bit-align the inter-polarized channel delaytime is to be set (step 11 (to be abbreviated as S11 in the figure, andsame rule will be applied to subsequent steps)). This instruction may befed to the delay control section 36 from the outside or the like, whenthe polarization multiplexed light transmitter is started up or when thesystem state is changed as a result that optical paths through which thepolarization multiplexed light is transmitted are changed-over in anoptical transmission system to which the polarization multiplexed lighttransmitter is applied. In the delay control section 36 received theabove instruction, a judgment of the instruction is performed (S12), andwhen the inter-polarized channel delay time is set to bebit-interleaved, the routine proceeds to step 13, whereas when theinter-polarized channel delay time is set to be bit-aligned, the routineproceeds to step 16.

When the bit-interleave setting is instructed, firstly, the delayamounts of the delay amount varying sections 21A and 21B are set atinitial values previously set corresponding to the bit-interleave (S13).Thus, the modulation signals delayed in accordance with the initialvalues by the delay amount varying sections 21A and 21B are fed to theoptical modulating sections 13A and 13B, and the signal lights(polarized channels) modulated in accordance with the delayed modulationsignals are combined by the polarization combining section 15, so thatthe polarization multiplexed light is generated. A part of thepolarization multiplexed light is branched by the optical branchingsection 31 as the monitor light, and the monitor light is fed to thepolarization interfering section 32, the photoelectric conversionsection 33, the signal processing section and the power measuringsection 35, in this sequence, so that the power of the alternate currentcomponent in the electric signal obtained by photo-electricallyconverting the polarization interfering light is measured, and themeasurement result is transferred to the delay control section 36 (S14).In the delay control section 36, the change in the measured power of thepower measuring section 35 is observed, and the delay amounts of thedelay amount varying sections 21A and 21B are feedback controlled sothat the measured power becomes minimal (S15). As a result, thepolarization multiplexed light set to be inter-leaved is transmitted instable from the output port OUT.

On the other hand, when the bit-align setting is instructed, firstly,the delay amounts of the delay amount varying sections 21A and 21B areset at initial values previously set corresponding to the bit-align(S16). Thus, the modulation signals delayed in accordance with theinitial values by the delay amount varying sections 21A and 21B are fedto the optical modulating sections 13A and 13B, and the signal lights(polarized channels) modulated in accordance with the delayed modulationsignals are combined by the polarization combining section 15 so thatthe polarization multiplexed light is generated. A part of thepolarization multiplexed light is branched by the optical branchingsection 31 as the monitor light, and the monitor light is fed to thepolarization interfering section 32, the photoelectric conversionsection 33, the signal processing section 34 and the power measuringsection 35, in this sequence, so that the power of the alternate currentcomponent in the electric signal obtained by photo-electricallyconverting the polarization interfering signal is measured, and themeasurement result is transferred to the delay control section 36 (S17).In the delay control section 36, the change in the measured power of thepower measuring section 35 is observed, and the delay amounts of thedelay amount varying sections 21A and 21B are feedback controlled sothat the measured power becomes maximal (S18). As a result, thepolarization multiplexed light set to be bit-aligned is transmitted instable from the output port OUT.

As described in the above, according to the polarization multiplexedlight transmitter in the first embodiment, the change in the power ofthe alternate current component in the electric signal obtained byphoto-electrically converting the polarization interfering light ismonitored by the delay control unit 30 of simple configuration disposedinside thereof. Thus, the pulse timing between the orthogonal polarizedchannels contained in the polarization multiplexed light can be changedflexibly at a high speed, based on the monitoring result. Therefore, itis possible effectively suppress transmission characteristicsdegradation of the polarization multiplexed light due to the change insystem state.

Next, there will be described specific practical examples correspondingto the first embodiment.

FIG. 14 is a block diagram illustrating a configuration of a practicalexample 1-1 of the polarization multiplexed light transmitter. Thispractical example 1-1 illustrates one example of specific configurationsof the polarization multiplexed light transmitter for transmitting theabove Pol-MUX_RZ-DQPSK signal light.

The polarization multiplexed light transmitter illustrated in FIG. 14splits a continuous light output from a semiconductor laser (LD) 111into two lights of which polarization states are mutually orthogonal bymeans of a polarization beam splitter 121, to feed one of the splitlights to a DQPSK modulator 131A and a RZ modulator 132A which arecascade connected to each other, while feeding the other light to aDQPSK modulator 131B and a RZ modulator 132B which are cascade connectedto each other.

The DQPSK modulator 131A is driven based on output signals from driveramplifiers 142A and 143A to which modulation signals DATA_A and DATA_Bgenerated in accordance with transmission data by a modulation signalgenerating circuit (MSGC) 141A are fed via a phase shifter 211A, toDQPSK modulate the input light from the polarization beam splitter 121.Further, the RZ modulator 132A is driven based on an output signal froma driver amplifier 145A to which a clock signal CLK_A synchronized withthe modulation signals DATA_A and DATA_B is fed via the phase shifter211A, and intensity modulates the DQPSK modulated light output from theDQPSK modulator 131A, to generate a RZ-DQPSK modulated light.

The DQPSK modulator 131B is driven based on output signals from driveramplifiers 142B and 143B to which modulation signals DATA_C and DATA_Dgenerated in accordance with transmission data by a modulation signalgenerating circuit (MSGC) 141B are fed via a phase shifter 211B, toDQPSK modulate the input light from the polarization beam splitter 121.Further, the RZ modulator 132B is driven based on an output signal froma driver amplifier 145B to which a clock signal CLK_B synchronized withthe modulation signals DATA_C and DATA_D is fed via the phase shifter211B, and intensity modulates the DQPSK modulated light output from theDQPSK modulator 131B, to generate a RZ-DQPSK modulated light.Incidentally, herein, a low frequency signal output from an oscillator363 is superimposed on the clock signal CLK_B generated in themodulation signal generating circuit 141B, as a dithering signal.

The DQPSK modulators 131A and 131B each comprises, for example, a firstMach-Zehnder (MZ) optical waveguide formed on a substrate having anelectro-optic effect, and second and third MZ optical waveguidesdisposed on a pair of branching arms of the first MZ optical waveguide.The input light from the polarization beam splitter 121 is branched intotwo lights by the first MZ optical waveguide, and the branched lightsare input respectively to the second and third MZ optical waveguides onthe branching arms. In the second and third MZ optical waveguides, theoutput lights from the driver amplifiers are applied to bias electrodesarranged along the branching arms, and refractive indexes of thebranching arms are changed due to an electric field generated by theapplication of the output lights so that the guided lights are phasemodulated in accordance with the transmission data. The phase modulatedlights output from the second and third MZ optical waveguides aremultiplexed by the first MZ optical waveguide, after one of the phasemodulated lights is fed to a phase shifter in which the phase thereof isshifted by π/2. Incidentally, to the bias electrodes and the phaseshifter on each of the second and third MZ optical waveguides, requiredbias voltages output from each of bias supply circuit 144A and 144B aresupplied.

The RZ modulating sections 132A and 132B each comprises, for example, aMZ optical waveguide formed on a substrate having an electro-opticeffect, and electrodes arranged along branching arms of the MZ opticalwaveguide. To the electrodes, the output signals from each of the driveramplifiers 145A and 145B are applied, and refractive indexes of thebranching arms of the MZ optical waveguide are changed due to anelectric field generated by the application of the output signals sothat the DQPSK modulated lights from the DQPSK modulators 131A and 131Bare intensity modulated (pulse carvered) in accordance with the clocksignals, and accordingly, RZ-DQPSK modulated lights are generated.Incidentally, to the bias electrode on each of the MZ opticalwaveguides, a required bias voltage output from each of bias supplycircuits 146A and 146B is supplied.

The RZ-DQPSK modulated lights output from the RZ modulating sections132A and 132B are fed to a polarization beam combiner 151 to be combinedin one light, so that a Pol-MUX_RZ-DQPSK signal light is generated. ThePol-MUX_RZ-DQPSK signal light is transmitted to the outside from theoutput port, and at the same time, a part thereof is branched by anoptical branching coupler 311 as a monitor light. This monitor light isfed, herein, to a polarizer 322 via a polarization angle regulator 321(corresponding to the first configuration example of the polarizationinterfering section 32 in FIG. 3), so that a polarization interferinglight obtained by making orthogonal polarized channels to interfere witheach other is generated. The polarization interfering light is convertedinto an electric signal by a light receiving element (PD) 331, and,thereafter, a direct current component thereof is eliminated by acapacitor 341, and further, an alternate current component thereof isrestricted to be within a required frequency band by a filter circuit(FIL) 342. Then, the electric signal passed through the filter circuit342 is fed to a power monitor 351 in which the power of the electricsignal is measured, and a measurement result is transferred to asynchronous detecting circuit 361.

In the synchronous detecting circuit 361, a dithering componentcontained in an output signal from the power monitor 351 issynchronously-detected using the low frequency signal from theoscillator 363, and a detection result is transferred to a delay controlcircuit 362. In the delay control circuit 362, a change in output signalof the synchronous detecting circuit 361 is observed, and delay amountsin the phase shifters 211A and 211B are feedback controlled inaccordance with a flowchart illustrated in FIG. 13 or the like.

According to the polarization multiplexed light transmitter in the abovepractical example 1-1, the inter-polarized channel delay time can bechanged flexibly at a high speed, according to the system state, andtherefore, it is possible to transmit in stable the Pol-MUX_RZ-DQPSKsignal light by which excellent transmission characteristics areobtainable.

Next, there will be described a further practical examples correspondingto the first embodiment.

FIG. 15 is a block diagram illustrating a configuration of a practicalexample 1-2 of the polarization multiplexed light transmitter.

The configuration of the polarization multiplexed light transmitterillustrated in FIG. 15 differs from that of the practical example 1-1illustrated in FIG. 14 in that the optical branching coupler 311 isomitted by utilizing a leaked light from the polarization beam combiner151 as the monitor light.

In the polarization beam combiner 151, for example when the RZ-DQPSKmodulated lights output from the RZ modulators 132A and 132B arecombined using a Y-branched waveguide, since a part of the polarizationmultiplexed light is leaked out from the Y-branched waveguide to thesubstrate, it is possible to utilize the leaked light as the monitorlight, by guiding the leaked light to an input port of the polarizationangle regulator 321 (for example, by disposing the polarization angleregulator 321 in the vicinity of a Y-branched portion on the substrateformed with the Y-branched waveguide, by omitting the polarization angleregulator 321 to dispose the polarizer 322 of which main axis isoptimized or the like). Further, for example when the RZ-DQPSK modulatedlights output from the RZ modulators 132A and 132B are combined using atwo-input two-output directional coupling waveguide, a light (equivalentto the leaked light) guided to an output port, which is not connected tothe output port OUT of the polarization multiplexed light transmitter,in two output ports of the directional coupling waveguide, may be fed tothe polarization angle regulator 321 as the monitor light.

By applying the above configuration, since the polarization beamcombiner 151 comprises both functions of the polarization combiningsection 15 and the optical branching section 31, it becomes possible torealize the polarization multiplexed light transmitter of furtherminiaturized and low cost.

Next, there will be described a still further practical examplecorresponding to the first embodiment.

FIG. 16 is a block diagram illustrating a configuration of a practicalexample 1-3 of the polarization multiplexed light transmitter.

The configuration of the polarization multiplexed light transmitterillustrated in FIG. 16 differs from that of the practical example 1-1illustrated in FIG. 14 in that a polarization combining device 326 isdisposed in place of the polarizer 322 and the monitor light output fromthe polarization angle regulator 321 and the leaked light from thepolarization beam combiner 151 are fed to the polarization combiningdevice 326, to be combined.

In the above configuration, in the polarization angle regulator 321, apolarization angle of the monitor light branched by the opticalbranching coupler 311 is regulated to be rotated by 90° relative to apolarization angle of the leaked light from the polarization beamcombiner 151. Then, the output light from the polarization angleregulator 321 and the leaked light from the polarization beam combiner151 are combined by the polarization combining device 326, so that theorthogonal polarized channels interfere with each other, andconsequently, the polarization interfering light is output from thepolarization combining device 326 to the light receiving element 331.

By applying the above configuration, it is possible to generate furtherefficiently the polarization multiplexed light. Thus, it becomespossible to increase monitoring intensity in the power monitor 351. Or,since required monitoring intensity can be realized even if the bandrestriction in the filter circuit 342 is set to a lower side, it becomespossible to achieve the further miniaturization and low cost performanceof the polarization multiplexed light transmitter.

Next, there will be described an even still further practical examplecorresponding to the first embodiment.

FIG. 17 is a block diagram illustrating a configuration of a practicalexample 1-4 of the polarization multiplexed light transmitter.

The configuration of the polarization multiplexed light transmitterillustrated in FIG. 17 differs from that of the practical example 1-1illustrated in FIG. 14 in that the polarization separating device 323,the polarization angle regulator 325 and the polarization combiningdevice 326 are disposed in place of the polarizer 322. The configurationof this polarization interfering section corresponds to the thirdconfiguration example of the polarization interfering section 32illustrated in FIG. 5.

In the above configuration, in the polarization angle regulator 321, thepolarization angle of the monitor light from the optical branchingcoupler 311 is regulated according to the main axis direction of thepolarization separating device 323, so that the polarized channels “x”and “y” are separated to be output respectively from output ports of thepolarization separating device 323. Then, the one polarized channel isdirectly fed to the polarization combining device 326, whereas the otherpolarized channel is fed to the polarization combining device 326 afterthe polarization direction thereof is rotated by 90° by the polarizationangle regulator 325. As a result, the light obtained by making thepolarized channels “x” and “y” to interfere with each other isgenerated, and the generated polarization interfering light is output tothe light receiving element 331 from the polarization combining device326. Incidentally, similarly to the configuration example illustrated inFIG. 6, the faraday rotating mirror (FRM) 327 may be disposed in placeof the polarization angle regulator 325. If the FRM 327 is used, it ispossible to achieve the further miniaturization of the polarizationmultiplexed light transmitter.

Next, there will be described a second embodiment of the polarizationmultiplexed light transmitter according to this invention.

FIG. 18 is a block diagram illustrating a configuration of thepolarization multiplexed light transmitter in the second embodiment.

Contrary to the above first embodiment in which the inter-polarizedchannel delay time is variably controlled in an electric signal(modulation signal) stage thereof by the delay amount varying sections21A and 21B, in the polarization multiplexed light transmitter in thepresent embodiment, the inter-polarized channel delay time is variablycontrolled in an optical signal stage thereof. To be specific, in placeof the delay amount varying sections 21A and 21B used in the firstembodiment, optical delay amount varying sections 22A and 22B aredisposed on optical paths between the optical modulating section 13A and13B, and the polarization combining section 15. Then, optical delayamounts in the optical delay amount varying sections 22A and 22B arefeedback controlled by the delay control section 36. Incidentally, otherconfigurations in the second embodiment except for the optical delayamount varying sections 22A and 22B are same as those in the firstembodiment, and accordingly, the description thereof is omitted here.

FIG. 19A is a block diagram illustrating a configuration of a practicalexample of the polarization multiplexed light transmitter correspondingto the second embodiment. In this practical example, in place of thephase shifters 211A and 211B used in the configuration of the practicalexample 1-1 illustrated in FIG. 14, optical delay devices 221A and 221Bare inserted on optical paths between the RZ modulator 132A and 132B,and the polarization beam combiner 151. The optical delay devices 221Aand 221B are feedback controlled by the delay control circuit 362,similarly to the practical example 1-1. Incidentally, herein, in theoptical delay device 221B, the low frequency signal from the oscillator363 is superimposed on the RZ-DQPSK modulated light output from the RZmodulator 132B.

According to the second embodiment as described above and the practicalexample thereof, similarly to the first embodiment and the practicalexample 1-1, since the delay time between the orthogonal polarizedchannels in the polarization multiplexed light can be changed flexiblyat a high speed, it is possible to effectively suppress the transmissioncharacteristics degradation due to the change in system state.

Incidentally, as the specific practical example of the secondembodiment, there has been described the configuration corresponding tothe practical example 1-1. However, it is possible to apply theconfigurations corresponding to the practical examples 1-2 to 1-4 to thesecond embodiment.

Further, in the practical examples corresponding to the first and secondembodiments described above, the description has been made applying theconfiguration provided with the RZ modulators 132A and 132B thatcorrespond to the DQPSK modulators 131A and 131B respectively. However,as illustrated in FIG. 19B for example, it is possible to apply aconfiguration which provides a common RZ modulator 132 for the DQPSKmodulators 131A and 131B.

In FIG. 19B, the RZ modulator 132 is arranged between the semiconductorlaser 111 and the polarization beam splitter 121. And optical delaydevices 221A′ and 221B′ are inserted on optical paths between thepolarization beam splitter 121 and the DQPSK modulators 131A, 131B. Inthe RZ modulator 132, the continuous light output from a semiconductorlaser 111 is intensity modulated according to clock signal CLK. A pulselight output from the RZ modulator 132 is split into two lights of whichpolarization states are mutually orthogonal by the polarization beamsplitter 121, then the split lights are given to the optical delaydevices 221A′ and 221B′ respectively. In the optical delay devices 221A′and 221B', similarly to the case illustrated in FIG. 19A, the pulsetiming between the orthogonal polarized channels is variably controlled.Here, similarly to the first embodiment, the timing of each drivingsignal of the RZ modulator 132 and DQPSK modulators 131A, 131B is alsocontrolled by the delay control circuit 362. Therefore, it becomespossible to simplify the configuration of the polarization multiplexedlight transmitter by applying the above-mentioned configuration of FIG.19B.

Next, there will be described a third embodiment of the polarizationmultiplexed light transmitter according to this invention.

In the first and second embodiment described above, the description hasbeen made on the case of measuring the power of the alternate currentcomponent in the electric signal obtained by photo-electricallyconverting the polarization interfering light to judge theinter-polarized channel delay time based on the change in the measuredpower. However, as indicated in the calculation results illustrated inFIG. 9, it is also possible to judge the inter-polarized channel delaytime based on the change in the waveform of the polarization interferinglight. Therefore, in the third embodiment, there will be described anapplication example corresponding to the above.

FIG. 20 is a block diagram illustrating a configuration of thepolarization multiplexed light transmitter in the third embodiment.

In FIG. 20, the polarization multiplexed light transmitter in thepresent embodiment, in place of the signal processing section 34 and thepower measuring section 35 used in the configuration of the firstembodiment illustrated in FIG. 1, a waveform measuring section 37 thatmeasures the waveform of the electric signal output from thephotoelectric conversion section 33, is disposed, to transfer ameasurement result of the waveform measuring section 37 to the delaycontrol section 36. Incidentally, other configurations in the thirdembodiment except for the waveform measuring section 37 are same asthose in the first embodiment, and therefore, the description thereof isomitted here.

In the above configuration, in the delay control section 36, theinter-polarized channel delay time is judged based on a change in thewaveform measured by the waveform measuring section 37. Similarly toFIG. 8 to FIG. 10 for example, if it is assumed that thePol-MUX_RZ-DQPSK signal light of 43 Gbps is transmitted from thepolarization multiplexed light transmitter, the waveform measured by thewaveform measuring section 37 is different due to the change in theinter-polarized channel delay time, as illustrated in calculationresults of FIG. 21 (FIG. 21 illustrates the cases where the delay timeis 0 bit time, ¼ bit time and ½ bit time). In these calculation results,for example by focusing on a change in peak amplitude of the measuredwaveform, it is possible to judge the inter-polarized channel delaytime. Namely, the peak amplitude of the measured waveform is mostlyincreased when the inter-polarized channel delay time is n bit time (nis integer), whereas the peak amplitude of the measured waveform ismostly decreased when the inter-polarized channel delay time is (2n+1)/2bit time.

Accordingly, when the inter-polarized channel delay time is set to bebit-interleaved, the delay control section 36 may feedback control thedelay amount varying sections 21A and 21B so that the peak amplitude ofthe measured waveform becomes minimal. Further, when the inter-polarizedchannel delay time is set to be bit-aligned, the delay control section36 may feedback control the delay amount varying sections 21A and 21B sothat the peak amplitude of the measured waveform becomes maximal.Furthermore, similarly to the first embodiment, it is also possible toset the inter-polarized channel delay time to be in between thebit-interleave and the bit-align.

FIG. 22 is a block diagram of a configuration of a practical example ofthe polarization multiplexed light transmitter corresponding to thethird embodiment. In this practical example, in place of the capacitor341, the filter circuit 342 and the power monitor 351 used in theconfiguration of the practical example 1-1 illustrated in FIG. 14 forexample, a waveform monitor 371 is disposed, so that the waveform of theelectric signal output from the light receiving element 331 is monitoredby the waveform monitor 371 and the monitored waveform is transferred tothe delay control circuit 362. Incidentally, it is possible to omit theoscillator 363 for superimposing the low frequency signal on themodulation signal and the synchronous detecting circuit 362, in the casewhere the waveform of the electric circuit is monitored.

According to the third embodiment as described above and the practicalexample thereof, similarly to the first embodiment and the practicalexample 1-1, it is possible to effectively suppress the transmissioncharacteristics degradation of the polarization multiplexed light due tothe change in system state, by monitoring the waveform of the electricsignal obtained by photo-electrically converting the polarizationinterfering light to control the inter-polarized channel delay timebased on the change in the monitored waveform.

Incidentally, as the specific practical example of the third embodiment,there has been described the configuration corresponding to thepractical example 1-1. However, the configurations corresponding to thepractical examples 1-2 to 1-4 are applicable to the third embodiment.Further, similarly to the second embodiment, the inter-polarized channeldelay time may be variably controlled in the optical signal stagethereof.

Next, there will be described a fourth embodiment of the polarizationmultiplexed light transmitter according to this invention.

In the first and second embodiments described above, the direct currentcomponent in the electric signal obtained by photo-electricallyconverting the polarization interfering light, is eliminated by thesignal processing section 34, and the inter-polarized channel delay timeis judged based on the change in the power of the alternate currentcomponent of which frequency band is restricted as needed. However, itis also possible to extract a frequency component at the baud rate or atn times the baud rate in the electric signal obtained byphoto-electrically converting the polarization interfering signal, tothereby judge the inter-polarized channel delay time based on a changein the power of the frequency component. Therefore, in the fourthembodiment, there will be described an application example correspondingto the above.

FIG. 23 is a block diagram illustrating a configuration of thepolarization multiplexed light transmitter in the fourth embodiment.

In FIG. 23, in the polarization multiplexed light transmitter in thefourth embodiment, in place of the signal processing section 34 used inthe configuration of the first embodiment illustrated in FIG. 1, asynchronous detecting section 38 that extracts the frequency componentat the baud rate (or the frequency component at n times the baud rate)from the electric signal output from the photoelectric conversionsection 33, is disposed, so that the power of the frequency componentextracted by the synchronous detecting section 38 is measured by thepower measuring section 35 and a measurement result is transferred tothe delay control section 36. Incidentally, other configurations in thefourth embodiment except for the synchronous detecting section 38 aresame as those in the first embodiment, and accordingly, the descriptionthereof is omitted here.

The synchronous detecting section 38 is fed with a signal having afrequency corresponding to the baud rate (or n times the baud rate) ofthe modulation signal generated by each of the modulation drivingsections 14A and 14B, and using this signal, synchronously detects theelectric signal from the photoelectric conversion section 33 to extractthe frequency component corresponding to the baud rate (or n times ofthe baud rate). Incidentally, although not illustrated in the figure, anarrow band-pass filter which includes the frequency corresponding thebaud rate (or n times the baud rate) within a pass-band thereof, may beinserted between the photoelectric conversion section 33 and thesynchronous detecting section 38 or may be disposed in place of thephotoelectric conversion section 33.

In the above configuration, in the delay control section 36, theinter-polarized channel delay time is judged based on the change in thepower measured by the power measuring section 35. Similarly to FIG. 8 toFIG. 10 for example, if it is assumed that the Pol-MUX_RZ-DQPSK signallight of 43 Gbps is transmitted from the polarization multiplexed lighttransmitter, the frequency component surrounded by each broken line incalculation results illustrated in FIG. 24 (an upper stage of FIG. 24 isthe frequency component corresponding to the baud rate, and a lowerstage of FIG. 24 is the frequency component corresponding to n timesbaud rate) is extracted by the synchronous detecting section 38. Whenthe inter-polarized channel delay time is changed (FIG. 24 illustratesthe cases where the delay time is 0 bit time, ¼ bit time and ½ bittime), the power of the frequency component is increased/decreased.Incidentally, the power measured by the power measuring section 35 isthe power obtained by combining a linear spectrum component at the baudrate (or n times the baud rate) with the frequency component in thevicinity thereof.

In the calculation results of FIG. 24, in the case where the frequencycomponent corresponding to the baud rate is extracted, the powermeasured by the power measuring section 35 is mostly increased when theinter-polarized channel delay time is 0 bit time, while being mostlydecreased when the inter-polarized channel delay time is ½ bit time.Further, in the case where the frequency component corresponding to twotimes the baud rate is extracted, the power measured by the powermeasuring section 35 is mostly decreased when the inter-polarizedchannel delay time is 0 bit time, while being mostly increased when theinter-polarized channel delay time is ½ bit time. Incidentally, focusingon only the linear spectrum component of two times the baud rate, thereis a feature in that the linear spectrum component is lost when theinter-polarized channel delay time is ¼ bit time.

Accordingly, in the case where the frequency component corresponding tothe baud rate is extracted, when the inter-polarized channel delay timeis set to be bit-interleaved, the delay control section 36 may feedbackcontrol the delay amount varying sections 21A and 21B so that themeasured power of the power measuring section 25 becomes minimal,whereas when the inter-polarized channel delay time is set to bebit-aligned, the delay control section 36 may feedback control the delayamount varying sections 21A and 21B so that the measured power of thepower measuring section 25 becomes maximal. On the other hand, in thecase where the frequency component corresponding to two times the baudrate is extracted, the delay control section 36 may feedback control thedelay amount varying sections 21A and 21B in accordance with relationsadverse to the above. Further, if the delay amount varying sections 21Aand 21B are feedback controlled so that the linear spectrum component oftwo times the baud rate is lost, it becomes also possible to set theinter-polarized channel delay time to ¼ bit time (an intermediate statebetween the bit interleave and the bit align).

FIG. 25 is a block diagram illustrating a configuration of a practicalexample of the polarization multiplexed light transmitter correspondingto the fourth embodiment. In this practical example, in place of thecapacitor 341 and the filter circuit 342 used in the configuration ofthe practical example 1-1 illustrated in FIG. 14 for example, asynchronous detecting circuit 381 is disposed, and in the synchronousdetecting circuit 381, the frequency component corresponding to the baudrate (or n times the baud rate) is extracted from the electric signaloutput from the light receiving element 331, and further, the power ofthe frequency component is monitored by the power monitor 351.

According to the fourth embodiment described above and the practicalexample thereof, similarly to the first embodiment and the practicalexample 1-1, it is possible to effectively suppress the transmissioncharacteristics degradation of the polarization multiplexed light due tothe change in system state, by extracting the frequency componentcorresponding to the baud rate (or n times the baud rate) from theelectric signal obtained by photo-electrically converting thepolarization interfering light to control the inter-polarized channeldelay time based on the change in the monitored power of the frequencycomponent.

Incidentally, as the specific practical example of the fourthembodiment, there has been described the configuration corresponding tothe practical example 1-1. However, it is also possible to apply theconfigurations corresponding to the practical examples 1-2 to 1-4 to thefourth embodiment. Further, similarly to the second embodiment, thepulse timing between the polarized channels may be variably controlledin an optical signal stage thereof.

Next, there will be described a fifth embodiment of the polarizationmultiplexed light transmitter according to this invention.

In each of the embodiments described above, in order to make theorthogonal polarized channels in the monitor light branched by theoptical branching section 31 to interfere with each other, it isnecessary to administrate the polarization state of the monitor lightusing the polarization angle regulator or the like. However, if suchadministration of the polarization state is to be performed with highprecision, it becomes necessary to perform the regulation of highquality or the like, but the realization thereof may not be readilyachieved. Therefore, in the fifth embodiment, there will be described anapplication example corresponding to the above.

FIG. 26 is a block diagram illustrating a configuration of thepolarization multiplexed light transmitter in the fifth embodiment.

In FIG. 26, in the polarization multiplexed light transmitter in thepresent embodiment, for the configuration of the first embodiment inFIG. 1, a polarization scrambling section 39 is disposed on an opticalpath between the polarization combining section 15 and the opticalbranching section 31, and the polarization state of the polarizationmultiplexed light combined by the polarization combining section 15fluctuates randomly by the polarization scrambling section 39.Incidentally, other configurations in the fifth embodiment except forthe polarization scrambling section 39 are same as those in the firstembodiment, and accordingly, the description thereof is omitted here.

In the above configuration, the monitor light of which polarizationstate fluctuates randomly is fed to the power measuring section 35 viathe polarization interfering section 32, the photoelectric conversionsection 33 and the signal processing section 34, and the power of thealternate current component in the electric signal obtained byphoto-electrically converting the polarization interfering light ismeasured by the power measuring section 35, and further, theinter-polarized channel delay time is judged based on the change in themeasured power. Similarly to FIG. 8 to FIG. 10 for example, if it isassumed that the Pol-MUX_RZ-DQPSK signal light is transmitted from thepolarization multiplexed light transmitter, when the frequency band ofthe alternate current component is restricted to the low band by usingthe low-pass filter of which 3 dB bandwidth is 300 MHz, the powermeasured by the power measuring section 35 indicates dependence, asillustrated in FIG. 27, on the inter-polarized channel delay time.

Accordingly, the delay control section 36 may feedback control the delayamount varying sections 21A and 21B so that the measured power becomesminimal, when the inter-polarized channel delay time is set to bebit-interleaved. Further, the delay control section 36 may feedbackcontrol the delay amount varying sections 21A and 21B so that themeasured power becomes maximal, when the inter-polarized channel delaytime is set to be bit-aligned.

According to the polarization multiplexed light transmitter in the fifthembodiment described above, the polarization state of the polarizationmultiplexed light is varied randomly by the polarization scramblingsection 39. Therefore, since the polarization state of the monitor lightdoes not need to be administrated when the orthogonal polarizationchannels in the monitor light are made to interfere with each other, itbecomes possible to readily realize a monitoring system.

Incidentally, in the fifth embodiment, one example has been described inwhich the polarization scrambling section 39 is arranged between thepolarization combining section 15 and the optical branching section 31.However, as illustrated in FIG. 28 for example, it is possible to obtaina similar functional effect by disposing the polarization scramblingsection 39 on an optical path between the branching port of the opticalbranching section 31 and the polarization interfering section 32.Further, one example has been described in which the polarizationscrambling section 39 is disposed to the configuration of the firstembodiment. However, it is also possible to apply the polarizationscrambling section 39 to the configurations of the second to fourthembodiment.

Further, in the respective embodiments described above and the practicalexamples corresponding to the respective embodiments, the descriptionhas been made using the calculation results of the Pol-MUX_RZ-DQPSKsignal light. However, the modulation format for the polarizationmultiplexed light in this invention is not limited to the above example.For example, similarly to the first embodiment, in the case where thepower of the alternate current component in the electric signal obtainedby photo-electrically converting the polarization interfering light ismeasured for a signal light obtained by polarization multiplexing aNRZ-DQPSK modulated light of which pulsed format is different from thatof the RZ-DQPSK modulated light, it can be verified that the change inthe measured power indicates dependence, as illustrated in calculationresults of FIG. 29, on the inter-polarized channel delay time.

Comparing the calculation results in FIG. 29 with the calculationresults in FIG. 12 for the RZ-DQPSK modulation format, the NRZ-DQPSKmodulation format has dependence basically same as that of the RZ-DQPSKmodulation format although the control sensitivity thereof is inferiorto that of the RZ-DQPSK modulation format, and accordingly, it isunderstood that the inter-polarized channel delay time can be judgedbased on the change in the measured power.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the inventionand the concepts contributed by the inventor to furthering the art, andare to be construed as being without limitation to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although the embodiment(s) of the presentinventions have been described in detail, it should be understood thatthe various changes, substitutions, and alterations could be made heretowithout departing from the spirit and scope of the invention.

1. A polarization multiplexed light transmitter which transmits apolarization multiplexed light containing a first optical signal and asecond optical signal of which polarization states are mutuallyorthogonal, comprising: a delay regulating unit configured to regulaterelative delay times of the first and second optical signals; and adelay control unit configured to make orthogonal polarization componentscontained in a monitor light obtained by taking out a part of thepolarization multiplexed light to interfere with each other, to generatea polarization interfering light, and configured to judge a delay timebetween the first and second optical signals based on a change in astate of an electric signal obtained by photo-electrically convertingthe polarization interfering light, and configured to control aregulation amount in the delay regulating unit so that the judged delaytime reaches a predetermined value.
 2. A polarization multiplexed lighttransmitter according to claim 1, wherein the delay control unitcomprises: an optical branching section that branches the part of thepolarization multiplexed light as the monitor light; a polarizationinterfering section that makes the orthogonal polarization componentscontained in the monitor light branched by the optical branching sectionto interfere with each other, to thereby generate the polarizationinterfering light; a photoelectric conversion section that converts thepolarization interfering light generated by the polarization interferingsection into the electric signal; a signal processing section thateliminates at least a direct current component from the electricalsignal converted by the photoelectric conversion section; a powermeasuring section that measures the power of the electric signal passedthrough the signal processing section; and a delay control section thatjudges the delay time between the first and second optical signals basedon a change in the power measured by the power measuring section, tocontrol the regulation amount in the delay regulating unit so that thejudged delay time reaches the predetermined value.
 3. A polarizationmultiplexed light transmitter according to claim 2, wherein the signalprocessing section restricts a frequency band of an alternate currentcomponent in the electric signal converted by the photoelectricconversion section.
 4. A polarization multiplexed light transmitteraccording to claim 3, wherein the signal processing section restrictsthe frequency band of the alternate current component in the electricsignal converted by the photoelectric conversion section, to a lowerside of frequencies corresponding to baud rates of the first and secondoptical signals.
 5. A polarization multiplexed light transmitteraccording to claim 3, wherein the signal processing section extracts afrequency component corresponding to the baud rates of the first andsecond optical signals or a frequency component corresponding to theintegral multiple of the baud rates of the first and second opticalsignals, from the electric signal converted by the photoelectricconversion section.
 6. A polarization multiplexed light transmitteraccording to claim 1, wherein the delay control unit comprises: anoptical branching section that branches the part of the polarizationmultiplexed light as the monitor light; a polarization interferingsection that makes the orthogonal polarization components contained inthe monitor light branched by the optical branching section to interferewith each other, to thereby generate the polarization interfering light;a photoelectric conversion section that converts the polarizationinterfering light generated by the polarization interfering section intothe electric signal; a waveform measuring section that measures awaveform of the electric signal converted by the photoelectricconversion section; and a delay control section that judges the delaytime between the first and second optical signals based on a change inthe waveform measured by the waveform measuring section, to control theregulation amount in the delay regulating unit so that the judged delaytime reaches the predetermined value.
 7. A polarization multiplexedlight transmitter according to claim 2, wherein the polarizationinterfering section includes a polarizer, and the monitor light branchedby the optical branching section is fed to the polarizer in a statewhere polarization directions of the orthogonal polarization componentscontained in the monitor light are deviated from a direction of a mainaxis of the polarizer.
 8. A polarization multiplexed light transmitteraccording to claim 2, wherein the polarization interfering sectionincludes a polarization separating device, and the monitor lightbranched by the optical branching section is fed to the polarizationseparating device in a state where polarization directions of theorthogonal polarization components contained in the monitor light aredeviated from a direction of a main axis of the polarization separatingdevice.
 9. A polarization multiplexed light transmitter according toclaim 7, wherein in the polarization interfering section, a deviationangle of one of the polarization directions of the orthogonalpolarization components contained in the monitor light relative to thedirection of the main axis of the polarization separating device is45°+360°×N (N is integer).
 10. A polarization multiplexed lighttransmitter according to claim 2, wherein the polarization interferingsection includes a polarization separating device and a polarizationcombining device; and the monitor light branched by the opticalbranching section is fed to the polarization separating device in astate where one of polarization directions of the orthogonalpolarization components contained in the monitor light is coincidentwith a direction of a main axis of the polarization separating device,and further, a light from one of two output ports of the polarizationseparating device and a light from the other output port, which isrotated by 90°, are combined by the polarization combining device.
 11. Apolarization multiplexed light transmitter according to claim 10,wherein the polarization interfering section rotates the polarizationdirection of the light from the other output port of the polarizationseparating device by 90° using a faraday rotating mirror.
 12. Apolarization multiplexed light transmitter according to claim 2, whereinthe polarization interfering section includes an optical amplifier whichamplifies the monitor light branched by the optical branching section,and a nonlinear element which receives the monitor light amplified bythe optical amplifier and a control light to rotate polarizationdirection of one of the orthogonal polarization components contained inthe monitor light by a nonlinear effect.
 13. A polarization multiplexedlight transmitter according to claim 2, further comprising; apolarization combining section that combines the first and secondoptical signals into one light, wherein the optical branching sectionguides a leaked light from the polarization combining section to thepolarization interfering section as a monitor light.
 14. A polarizationmultiplexed light transmitter according to claim 1, further comprising:a polarization separating section that separates an output light from alight source section into two lights of which polarization states aremutually orthogonal; a first optical modulating section that modulatesone of the lights separated by the polarization separating section togenerate the first optical signal; a first modulation driving sectionthat generates a first modulation signal for driving the first opticalmodulating section; a second optical modulating section that modulatesthe other light separated by the polarization separating section togenerate the second optical signal; a second modulation driving sectionthat generates a second modulation signal for driving the secondmodulating section; and a polarization combining section that combinesthe first and second optical signals generated by the first and secondoptical modulating section into one light, wherein the delay regulatingunit makes relative delay times of the first and second modulationsignals output from the first and second modulation driving sections tobe variable.
 15. A polarization multiplexed light transmitter accordingto claim 1, further comprising: a polarization separating section thatseparates an output light from a light source section into two lights ofwhich polarization states are mutually orthogonal; a first opticalmodulating section that modulates one of the lights separated by thepolarization separating section to generate the first optical signal; afirst modulation driving section that generates a first modulationsignal for driving the first optical modulating section; a secondoptical modulating section that modulates the other light separated bythe polarization separating section to generate the second opticalsignal; a second modulation driving section that generates a secondmodulation signal for driving the second modulating section; and apolarization combining section that combines the first and secondoptical signals generated by the first and second optical modulatingsection into one light, wherein the delay regulating unit makes relativedelay times of the first and second optical signals generated by thefirst and second modulating sections to be variable.
 16. A polarizationmultiplexed light transmitter according to claim 1, wherein the delaycontrol unit comprises a polarization scrambling section that scramblesdirections of the orthogonal polarization components contained in thepolarization multiplexed light.
 17. A control method of a polarizationmultiplexed light transmitter which transmits a polarization multiplexedlight containing a first optical signal and a second optical signal ofwhich polarization states are mutually orthogonal, the control methodcomprising: taking out a part of the polarized multiplexed light; makingorthogonal polarization components contained in the taken out monitorlight to interfere with each other, to generate a polarizationinterfering light; judging a delay time between the first and secondoptical signals based on a change in a state of an electric signalobtained by photo-electrically converting the generated polarizationinterfering light: and controlling relative delay times of the first andsecond optical signals so that the judged delay time reaches apredetermined value.
 18. A polarization multiplexed light transmitteraccording to claim 8, wherein in the polarization interfering section, adeviation angle of one of the polarization directions of the orthogonalpolarization components contained in the monitor light relative to thedirection of the main axis of the polarization separating device is45°+360°×N (N is integer).